The economies associated with conventional part production methods are generally, related directly to the quantity of parts to be produced and the desired material characteristics of the finished parts. For example, large scale manufacture casting and extrusion techniques are often cost effective, but these production methods are generally unacceptable for small quantities--i.e. replacement parts or prototype production. Many such conventional part production methods require expensive part specific tooling. Even powder metallurgy requires a die for shaping the powder, making powder metallurgy unattractive as a method for producing a small number of parts.
Where only a small number of parts are desired, conventional production methods involving a subtractive machining method are usually used to produce the desired part. In such subtractive methods, material is cut away from a starting block of material to produce a more complex shape. Examples of subtractive machine tool methods include: milling, drilling, grinding, lathe cutting, flame cutting, electric discharge machine, etc. While such conventional machine tool subtractive methods are usually effective in producing the desired part, they are deficient in many respects.
First, such conventional machine tool subtractive methods produce a large amount of waste material for disposal. Further, such machine tool methods usually involve a large initial expense for setting up the proper machining protocol and tools. As such, the set-up time is not only expensive, but relies a great deal on human judgment and expertise. These problems are, of course, exacerbated when only a small number of parts are to be produced.
Another difficulty associated with such conventional machining techniques involves tool wear - which not only involves the cost of replacement, but also reduces machining accuracy as the tool wears. Another limit on the accuracy and tolerance of any part produced by conventional machining techniques is the tolerance limits inherent in the particular machine tool. For example, in a conventional milling machine or lathe, the lead screws and ways are manufactured to a certain tolerance, which limits the tolerances obtainable in manufacturing a part on the machine tool. Of course, the tolerances attainable are reduced with age of the machine tool.
The final difficulty associated with such conventional machine tool subtractive processes is the difficulty or impossibility of making many part configurations. That is, conventional machining methods are usually best suited for producing symmetrical parts and parts where only the exterior part is machined. However, where a desired part is unusual in shape or has internal features, the machining becomes more difficult and quite often, the part must be divided into segments for production. In many cases, a particular part configuration is not possible because of the limitations imposed upon the tool placement on the part. Thus, the size and configuration of the cutting tool do not permit access of the tool to produce the desired configuration.
There are other machining processes which are additive, for example, plating, cladding, and some welding processes are additive in that material is added to a starting substrate. In recent years, other additive-type machining methods have been developed which use a laser beam to coat or deposit material on a starting article. Examples include U.S. Pat. Nos. 4,117,302; 4,474,861; 4,300,474; and 4,323,756. These recent uses of lasers have been primarily limited to adding a coating to a previously machined article. Often such laser coating methods have been employed to achieve certain metallurgic properties obtainable only by such coating methods. Typically, in such laser coating methods the starting article is rotated and the laser directed at a fixed location with the coating material sprayed onto the article so that the laser will melt the coating onto the article.
The above-referenced U.S. Pat. Nos. 4,944,817 and 4,863,538, as well as U.S. Pat. No. 4,938,816, issued Jul. 3, 1990, and PCT publication WO 88/02677, published Apr. 21, 1988, all of which are incorporated herein by this reference, describe a method of producing complex parts directly from a CAD data base which is not subject to the above-described limitations of the various subtractive and additive methods; this new method will be referred to hereinbelow as the "selective beam sintering" or "selective laser sintering" processes. The selective laser sintering process is particularly advantageous in forming prototypes for parts which may subsequently be mass produced, for example by investment casting, or by the use of tooling.
To provide a part having the necessary strength, stability, and integrity to meet the mechanical and temperature stress requirements of its end use, it is of course desirable to form parts of high melting point materials. Accordingly, the formation of ceramic parts by selective beam sintering is desirable. However, many ceramic powders have sintering or melting temperatures which are sufficiently high that selective sintering or melting by a directed energy beam, such as a laser, while still maintaining high dimensional resolution and close tolerances, is not easily achievable at this time.
By way of further background, the above-referenced copending applications Ser. No. 624,419 filed Dec. 7, 1990, and Ser. No. 657,151 filed Feb. 19, 1991, each describe methods and material systems for utilizing composite powders to form high temperature materials by way of the selective beam sintering processes, such high temperature materials including intermetallics and ceramics. The materials formed according to these methods and systems include those with higher melting points than in the constituent powders, thus allowing relatively low power lasers to form high temperature materials.
It is an object of this invention to provide a method of producing a part by the application of energy to selected portions of a powder layer, where the part is defined by the reaction of the powder material with gases in the atmosphere.
It is a further object of this invention to provide such a method where the shape and dimensions of the part are defined by such a method performed in layerwise fashion.
It is a further object of this invention to provide such a method where the part is formed of a composite material.
It is a further object of this invention to provide such a method where the powder includes multiple materials, so that the part is formed of a composite material.
It is a further object of this invention to provide such a method wherein the part being formed densifies as a result of the reaction.
It is a further object of this invention to provide such a method where the part formed has a higher melting point than that of the constituent powder.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification, together with the drawings.